The invention relates to a system comprising: an encoder the magnetic track of which has an alternation of North and South magnetic poles separated by transitions in Archimedean spiral, a rotation sensor able to detect the periodic magnetic field emitted by the encoder using a plurality of magnetic sensitive elements, distributed angularly along the magnetic track. Each magnetic sensitive element delivers a signal representative to the rotation of the encoder. The sensor further comprises a device for subtracting the signals (V1, V2) delivered by two sensitive elements forming therebetween an angle γ that is such that: 0.55π